Originally posted by F Thumb ...though I'm still trying to talk him into squeezing 4ohms into each element for even more heating surface (Steve's afraid at that point the coils become too tight - I'm going to be the test dummy).
...

I think I can help there - I've already been the test dummy! My elements are too close together, and the life is much shorter than it should be.

My furnace was designed "on the ragged edge" w.r.t. elements. The origninal Lauchner 40lb furnace uses 8Ga. elements that look like garage door springs. 1 groove per brick, so each element is 88" long, and wired in series.

With a local element maker in my neighborhood, I chose to try smaller elements - 15Ga (not 14Ga as I've previously said).

Like the original Lauckner design, I only put 1 groove per brick, for 88" of length per element. To get the necessary wire length with the smaller guage, the coils are almost touching (stretch is about 1.5-1).

As a result, I'm only going to get a little over 1000 hours from this set of elements.

On my next element change (iminent) I plan to cut an extra set of grooves (or maybe 2), increasing the stretch to more optimum levels. I may also go to 14Ga wire. Both should extend element life significantly.

In hindsight, I should have put more grooves per brick in from the start, but we all have to learn. Besides, this process is fun, and I've gotten some awesome clear glass out of the deal.

Richard, from what I have seen so far it seems that the fatter (lower guage wire) you use the longer it last because the chemical reaction starts killing the wire from the surface of the wire inwards and eventually gets to the point where you have a very thin(effective diameter/guage) conductor surrounded by oxide. Is this correct?

My question however is what would be the difference in Temp. between the glass itself and the thermocouple reading for a gas fired furnace and a wire furnace(wire around the pot).
I wonder if surrounding the pot with wire is more effective than gas. Brice Is that what you were leading up to? That wire can give more heat to the glass because it surrounds the glass?
If you placed your thermocouple on the side like in the electric furnace would you see the same temp as in your gas furnace with the thermocouple on top. Not sure where you would shoot the flame for a side thermocouple reading. Dont know much about gas except I can light Glory hole ok now and it gets hot ok.

Many studies have shown that the most effective transfer of heat both radiation and convection is direct convection/glass, radiation/glass.
So electric elements should see the glass for best results. On the down side if this increases the size and weight of the furnace you might end up with more kW/h.

Mine started life as the basic Mark Lauckner 15 pound furnace. 14 gauge 24 ohm element for 10 amp draw on 240V. I have heard to expect about 6-9 months effective element life, elements on 24/7 but I always seem to spill a bit of batch on the elements or something equivalent so no personal "best mileage" on an element yet. Even with the elements in good shape it's a bit underpowered anyway for doing something like c-balls where you're lifting the lid every few minutes.

Major changes over time:

Changing from mechanical relay to SSR and one second firing times

Making the square chamber into a round one after cracking a pot from it being too close to the elements at four points in the square configuration.

Went to one layer of grooved firebrick inside to hold the elements and the rest packed with scrap frax from ceramic fiber engineering at 25 cents per pound. Much better insulating this way.

Best to design in enough groove length so you can stretch the element at least twice it's close coiled length.

Think about watt loading. I believe it was 8-12 watts per sq. in. was the desired range.

Maybe think about wiring elements in parallel if space is going to be an issue.

I was very focused on getting even heat on the pot but as I learn more and some very experienced people teach me more things I'm thinking that uneven heat on the pot to get convection movement mixing in the glass would be a very good thing. This would be hard to do with wire due to space constraints and I'm not sure how much uneven heating a pot will endure. I'm thinking the uneven heat will work if the heat source is far enough away, as in around 2", but there goes the economy from a smaller furnace chamber.

I think there may be some economy in using 14 to 11 gauge wire instead of the 8 gauge "garage door springs" ( I love that description!). Those big ones seem to lay over and short out long before their smaller wound/thinner gauge cousins. Think Denver problems.
14 gauge is probably the minimum thickness of wire that's effective.

After waiting around for my current furnace to recover temp. during fast production any future ones will go by the old "one amp per pound of glass" rule of thumb.

[quote]Originally posted by Durk Valkema
[b]Many studies have shown that the most effective transfer of heat both radiation and convection is direct convection/glass, radiation/glass.
So electric elements should see the glass for best results.

That is a pretty major flaw in most of the electric furnaces with the exception of Denver. The surface of the glass is always the coldest and you're heating through the pot which is inefficient. I use the "thermal flywheel" ring too and I think its aa good a design as you can have with wire because it keeps down the chance of cullet popping on to the elements. I have about 1/2" between the pot and the ring. I have Steves elements too and they've been going for about three months with no failure yet. I've been melting cullet and last time I started up I decided to just run it at 2150 24/7. On the theory that running a bit high all the time is better than juymping all over the place. I do have trouble fining out my cullet though.

ps. one amp per pound of glass? Thats nuts! I have my scr dialed down to 34 amps and thats fine for me, charging recovery @ 2150 is around 20 minutes for 25 pounds of cullet. In a 100lb pot.

pss. I mean its nuts for it to be a rule of thumb. Melting two pounds with two amps is just as absurd as melting two hundred pounds with two hundred amps. But between say, 30 to 60 pounds it might make more sense. But a 60 pound furnace running on 60 amps is still pretty overpowered.

Originally posted by Steve Stadelman There comes a point where describing melting energy in pounds becomes pretty non-descriptive.

A 200lb pot is almost the same size as a 300lb pot, same for 80 and 100.

What always makes the difference is the size of the furnace itself, watts per cubic foot will probably be the best standard.

Ok, I understand that...what's the "rule of thumb" for watts per cubic foot.

I know a more accurate way of determining the wattage would be to determine that actual heat loss of a given furnace, and that will give you it's idle point, plus whatever you need to increase the temp...all of which is determined by the thermal mass of the whole thing, plus the thermal mass of the glass to be melted....now I have a headache...and I have to design one of these this year, it's gonna be a blast, I can see it now!

The one amp per pound assumes 240V,and is definitely a small melter (probably up to 40-50 lbs.)only sort of concept.

Any of these amps per pound or watts per cubic foot sort of approaches might be tempered by the thought that you may want to be able to gain heat quickly at some point. You don't have to use all that capacity all the time but it's sure nice to have it there when you need it.
You will also have different results depending on how much/what sort of insulation and furnace mass.

I think its pretty straitforward; you stuff as much power into it as space will allow, then you use your current or output control to dial back the current as much as possible to give the lowest watt loading you can live with.

Originally posted by David Williams I think its pretty straitforward; you stuff as much power into it as space will allow, then you use your current or output control to dial back the current as much as possible to give the lowest watt loading you can live with.

::: DingDingDingDingDing :::

We have a WINNER! "Stuff as much power into it as space will allow, then you use your current or output control to dial back the current as much as possible to give the lowest watt loading" is the answer we were looking for!

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